1. Field of the Invention
Embodiments in accordance with the present disclosure are directed to photomasks.
2. Description of the Related Art
Modern photolithography equipment is based on optical lithography, which uses optics to accurately project and expose a pattern in a photomask (or reticle) onto a photoresist-covered wafer. More specifically, the photolithography process can begin with the formation of a photoresist layer over the top surface of a semiconductor substrate or wafer. The photomask may be placed between a radiation source (e.g., light) and a projection lens system. The pattern produced by the radiation transmitted through the photomask may be focused to generate a reduced mask image on the wafer. The focusing and reduction of the mask image pattern is typically done using the projection lens system, which contains one or more lenses, filters, and/or mirrors. The radiation passing through clear regions of the photomask exposes the underlying photoresist layer. Depending upon the photoresist layer composition, the exposed portions of the photoresist can either become soluble or insoluble. Portions of the photoresist are removed to create a photoresist mask that is then used to remove portions of underlying structural layers within the wafer. The end result is a desired pattern formed on the wafer. Typically, multiple different photomasks are used to create different patterns in one or more layers on the wafer.
The resolution achieved through photolithography depends, in part, on the wavelength and coherence of the radiation source, as well as the numerical apertures (NA) of the lens within the photolithography system. Advanced photolithography uses optical enhancement techniques, such as “assist features” to improve the resolution of smaller features. Assist features are not intended to appear in the pattern developed in the resist but are provided in the photomask to take advantage of optical effects so that the developed image in the resist more closely resembles the desired pattern. Assist features are generally “sub-resolution” meaning that they are smaller in at least one dimension than the smallest feature in the photomask that will be resolved on the wafer. Note that because the pattern in the photomask may be projected with a magnification of less than 1, the assist feature in the photomask may have a physical dimension larger than the smallest feature on the wafer.
Despite advancements in using photomasks in a lithographic projection system to accurately print features on a wafer, further advancements are desired.